Lamp ballasts for fluorescent lamps or gas discharge lamps normally have a half bridge circuit and a series resonant circuit which is connected to the half bridge circuit and can be connected to the fluorescent lamp. In this case, the half bridge circuit is used to excite the series resonant circuit and for this purpose produces an AC voltage from a DC voltage which is applied across the half bridge.
A starting phase of a lamp ballast includes a preheating phase and an ignition phase for starting the lamp. During the preheating phase, incandescent filaments in the lamp are heated by setting an AC voltage frequency, which is referred to as the excitation frequency in the following, such that it is above the resonant frequency of the series resonant circuit. During the ignition phase, the excitation frequency is reduced increasingly in the direction of the resonant frequency of the resonant circuit with the aim of increasing the voltage across the fluorescent lamp as a result of a resonant peak to such an extent that a starting voltage for the lamp is reached, and that the lamp starts. During an operating phase after the lamp has been started, the excitation frequency can therefore be reduced further again.
During the ignition phase, one aim in this case is to ensure that the voltage across the lamp can rise up to the value of the starting voltage. On the other hand, another aim is to ensure, for safety reasons, that the voltage does not continue to rise indefinitely, for example when the lamp does not start because of a defect or when no lamp is connected to the resonant circuit. For this purpose, U.S. Pat. No. 6,525,492 proposes that a current through the half bridge be detected, and that the half bridge be switched off immediately when the current exceeds a predetermined threshold value.
For cost reasons the coil of the resonant circuit is frequently of such a size that it is already operating close to its magnetic saturation when the lamp voltage is in the region of the starting voltage. As is known, the effective inductance of a coil decreases when it changes to the saturation range. If an excitation frequency at which the coil starts to enter saturation is reached during the starting process, then the resonant frequency of the series resonant circuit increases because of the decreased inductance of the coil, and the margin between the instantaneous excitation frequency and the resonant frequency decreases. If the excitation frequency remains constant, the voltage continues to rise, the coil goes further into saturation, and the resonant frequency becomes even closer to the instantaneous excitation frequency. This positive-feedback effect that has been explained can result in instabilities in the setting of the starting voltage.
For these and other reasons, there is a need for the present invention.